The Section on Neuroanatomy studies the functional organization of the cerebral cortex and basal ganglia. Connections between functionally defined cortical areas provide neuroanatomical substrates of cognition, including purposive movement. We suggest that these cortico-cortical connections are represented in the organization of cortical inputs to the striatum, the basal ganglia target of the cortex. How the striatum processes this input, which is fed back to the cortex, is the subject of our work. These studies use neuroanatomical techniques. including axonal tract tracing. immunohistochemistry, and in situ hybridization histochemistry to examine cerebral cortical and basal ganglia connections, specifically the organization of cortical inputs to the striatum, and to characterize striatal output neurons in terms of their connections and expression of neurotransmitter receptors, signal transduction systems, and neurotransmitters/ peptides. Processing of cortical inputs to the striatum is studied with pharmacologic manipulations of dopamine and acetylcholine receptor-mediated gene regulation of connectionally and neurochemically defined striatal iatal neurons with quantitative in situ hybridization. As a working hypothesis, it is proposed that the basal ganglia process cortical inputs to either facilitate or disfacilitate feedback to the frontal cortex. This is thought to involve a balance between antagonistic striatal output pathways, which we have shown to be oppositely affected by dopamine. Our studies have provided insights into the cellular mechanisms involved in the disruption of the normal balance between these pathways that occurs in Parkinson's disease. Moreover, it is suggested that disordered cognition in schizophrenia may be related to corticocortical and corticostriatal dysfunction.